Our laboratory has been involved in the generation of monoclonal antibodies that are reactive with human mammary tumor cells. Screening of monoclonal antibodies involves determining reactivity with live and fixed mammary cell lines in culture as well as with extracts of mammary tumor tissues. We have further characterized several different monoclonal antibodies, using the immunoperoxidase technique, on the basis of their reactivity to tissue sections of primary and metastatic breast lesions and their lack of reactivity of normal mammary and non-mammary tissues of the same individual. We have also been able to characterize the different monoclonal antibodies on the basis of which mammary tumors are reactive, and on their pattern of staining. This technique has also enabled us to further define the antigenic heterogeneity that exists among different mammary tumors and within a mammary tumor of a given individual.